There is an urgent demand for capacitors that can store high amounts of electrical energy for uses that include vehicles, off-peak power generation, fuel cells, space exploration, and military applications. The principles of energy storage in capacitors have been well understood for some time. The key parameters determining energy density are the following:                The permittivity (dielectric constant) of the dielectric and its dependence on applied DC voltage        The maximum field that the dielectric can sustain without breakdown, and        The packaging efficiency of the dielectric        
The energy stored per unit volume of dielectric, J, is given by:
                    J        =                              ∫            0            Eb                    ⁢                                    ɛ              0                        ⁢                                          ɛ                r                            ⁡                              (                E                )                                      ⁢            E            ⁢                          ⅆ              E                                                          (        1        )            
where ∈0 is the permittivity of free space, ∈r(E) is the relative permittivity of the dielectric, E is the electrical field, and Eb is the dielectric breakdown strength (the highest electric field that the dielectric material can withstand). If, and only if, the dielectric is linear, i.e. the polarization increases linearly with applied field, equation (1) can be simplified to
                    J        =                              1            2                    ⁢                      ɛ            0                    ⁢                      ɛ            r                    ⁢                      E            b            2                                              (        2        )            
It is well known that the dielectric materials for these capacitors art typically chosen from materials characterized by a combination of high dielectric permittivity and high break-down strength. Unfortunately, materials with very high break-down strengths, such as polymers, tend to have low dielectric permittivity. Efforts to increase the permittivity by loading polymers with powders of high permittivity ceramics generally result in degraded break-down strength. Conversely, dielectrics with high permittivity such as those based on barium titanate usually have relatively low break-down strength and, moreover, the permittivity is often non-linear, being strongly suppressed by the application of high electric fields.
Early work on ceramic dielectrics indicated that a near linear dielectric based on strontium titanate with a permittivity of about 225 had lower energy storage capability than a high permittivity dielectric based on barium titanate, even when the dependence permittivity on field was taken into account. The same conclusion was reached when the energy storage capability of the strontium titanate dielectric was compared with anti-ferroelectric ceramic dielectrics based on lead zirconate.
However, recent research has indicated that energy storage in sintered pellets of titanium dioxide, a linear dielectric with a permittivity of about 125, can have high energy storage capacity (ca. 14J/cc) when the grain size is kept small (<500 nm) during sintering in an oxygen atmosphere, because breakdown voltages as high as 140V/μm can be achieved.
On the other hand, manufacturing multilayer ceramic capacitors using a titanium dioxide dielectric with fired grain size <500 nm presents numerous processing difficulties. Multilayer ceramic capacitors are usually constructed by casting and then drying a slurry of dielectric powder, organic binder and a solvent to form a flexible “green” tape. A metal paste, or ink, consisting of metal powder, an organic resin and a solvent, is applied to one side of the tape, usually by a screen printing process, and then layers of the metalized tape are stacked and laminated to form a monolithic body in which alternate metal layers respectively have a common polarity. This monolithic structure must be then fired to sinter the ceramic dielectric and bond the inner metal layers to the ceramic. In the case of capacitors containing titanium dioxide as a dielectric, problems can be expected when binder materials are removed from the ceramic and from the electrode layers because of the strong tendency of titanium dioxide to become semiconducting if the combusting organics lower the level of oxygen within the capacitor during processing. In addition, there can be chemically incompatibility between the dielectric material and the metal in the electrodes and mechanical incompatibility due to differences in shrinkage of the ceramic and metal layers during firing.
Thus, there remains a need for multilayer ceramic capacitors exhibiting both high dielectric permittivity and high breakdown strength, as well as for a way of fabricating the same. The invention addresses these needs.